1. Field of the Invention
The present invention relates to electric storage batteries and, more particularly, to an improved and unique reversible electric cell of the type having a soluble cathode, an insoluble positive electrode and an electrolyte comprising an aqueous solution of a metal halide.
2. Description of the Prior Art
As is well known in the art, electric storage batteries have been proposed which have high specific energy and a coefficient of utilization of active anodic and cathodic substances nearing unity. In general, such batteries are made up of a number of cells, each cell containing a metallic cathode, an aqueous electrolyte of a salt of a cathodic metal, with which the cathode reacts to form a soluble salt, and an anode dissolved in an electrolyte. The use of zinc or cadmium, as a metal for the cathode, and a halogen such as chlorine, bromine or iodine, as the dissolved anode, has been proposed. In such a system the aqueous electrolytic salt would be composed of a halide of the soluble cathodic metal, e.g., zinc bromide.
Such batteries have a number of advantages including a high specific energy in relation to the active mass. For example, in the case of the zinc-bromine pair, the electromotive force is about 1.82 volts with the theoretical specific energy being about 436 Wh/kg.
While a number of electric storage batteries have been proposed and have, to some extent, achieved commercial acceptance, known batteries suffer from certain disadvantages. For example, in such known batteries the aqueous solution at the end of the charging process (with the circuit open) contains a high percentage of halogen which attacks the cathodic metal, e.g., the zinc, chemically and thus causes autodischarge of the cell.
This disadvantage may be abdicated by preventing the electrolytic solution from entering into contact with the zinc when the latter is not required to furnish an electric charge. In addition, it has been proposed to introduce, in the recycling circuit of the electrolyte, an organic solvent which is not soluble in water and which is capable of retaining the halogen which has been liberated during the charging process. While such solutions have been proposed, it has been discovered that they are not always efficient and further require an increase in the mass of material not necessary to the electro-chemical reaction. In addition, auxiliary devices whose weight causes a diminuation of the value of the specific energy obtainable, are required. For example, in the case of the zinc-bromine pair, the theoretical specific energy drops from 436 Wh/kg to a real or actual value of less than 100 Wh/kg.
An example of a U.S. patent which discloses a battery or electric cell of the type described above is U.S. Pat. No. 2,566,114 to Bloch, which issued Aug. 28, 1951. In this patent the halogen is sought to be retained by the means of an alkylammonium halide, such as tetraethylammonium bromide. The electric cell disclosed by this patent suffers from the disadvantage that when the element is charged, i.e., when the alkylammonium halide has absorbed its maximum of halogen, the halogen is in contact with the zinc, which immediately causes an autodischarge of the element. Also, because of the formation of differences in density of the electrolyte, a lack of uniformity in the deposits is developed during the charge/discharge cycle which can damage the interior of the element.